It is known in the art to realize such synchronizing devices wherein a couple of first cranks, reciprocally parallel, and a couple of second cranks, reciprocally parallel too, connect the seat frame, respectively in its front and rear portion, with the chair base, thereby forming, with the same seat frame, a couple of four-bar linkages, each of them being disposed on a side of the chair. Such a synchronizing devices provide as well that the seat frame is cinematically constrained to such four-bar linkages, preferably by a leverage having a fulcrum on the base of the chair itself.
It has to be noticed that herein below reference will be made to such a synchronizing devices having one four-bar linkage only, composed of two cranks hinged in the chair base and, respectively, in the front and rear portion of the seat frame, both because it is theoretically possible the use of one four-bar linkage of such a type only, and because in such a way the description will be simplified.
For example, the French Patent FR 2045120, in the name of DUPART, teaches to realize such a synchronizing device for tilting the seat frame and the chair back frame by a four-bar linkage provided with only two cranks hinged between the base and the frame of the seat, and a lever with two arms, having a fulcrum on the same base, and wherein an arm is integral with the chair back frame and on the contrary the other arm is acting onto the rear portion of the seat frame, raising or pulling it down as a function of the angle and the tilting direction of the chair back.
Such a solution, although mechanically easy, however revealed not much comfortable for the user, because this latter is caused to exercise a considerable force onto the chair back for opposing to his/her own weight acting on the seat, due to the necessary reduction of the dimension of the afore said lever with two arms.
In addition, the DUPART Patent, because of the geometry of the described synchronizing device, does not provide, while varying the chair back tilt angle, the contemporaneous raising, or pulling down, of the front portion and the rear portion of the seat in such a way that this raising or pulling down of the front portion of the seat would be greater than the raising or pulling down of the rear portion of the seat.
The German Application DE 37 35 256, in the name of NIKOV, describes a synchronizing device of the tilt of the back and seat of a chair wherein the four-bar linkage comprises two cranks, one in the front and the other in the rear, that are hinged between the seat frame and the base of the chair, and wherein the seat frame is integral with the rear crank of such a four-bar linkage.
The front and rear cranks are tilted in different directions relatively to the vertical, that is if the front crank is tilted with an acute angle in the anticlockwise direction relatively to the vertical, then the rear crank is tilted with an acute angle in the clockwise direction relatively to the vertical, and/or vice versa.
Such a solution, not so much appreciated by all users, provided that when the tilt of the chair back changes, the front portion of the seat will raise and contemporaneously the rear portion of the seat will pull down, or vice versa.
In addition, the problem of the difficulties in tilting the chair back, because of the weight acting onto the seat and the lever arm not excessively long, would not seem to be solved by the NIKOV's device.
The International Patent Application WO 2008/067947, in the name of SATO OFFICE, refers to a device for synchronizing the tilt of the seat frame and the back frame of a chair comprising a four-bar linkage, wherein the two cranks are hinged to the chair base and, respectively, to the front and rear portions of the seat frame, and a lever with two arms having a fulcrum on the end extension of the rear crank. One of the two arms of such a lever is constrained, by means of a runner, to the front portion of the seat frame, whereas the other arm is integral with the back frame.
Also this SATO OFFICE's solution provides that the rotation of the chair back would cause the contemporaneous raising of the front portion of the seat and the pulling down of the rear portion of the same seat, with an effect not always appreciated by the user.
In addition, the use of a runner, made of a pin sliding into an extended eyelet, as a constrain between the afore said lever and the seat frame, will impose a high working and assembling precision of the SATO OFFICE's device.
It is therefore an object of the present invention to realize a device for synchronizing the tilt of the back and seat of a chair, or any other seating means, not presenting the drawbacks of the known prior art, and then resulting mechanically simple and comfortable for the user.
It is another object of the present invention to provide such a synchronizing device allowing the user, relatively to his/her own weight, to simply and effectively adjust the tilt of the chair back, and then of the seat, in such a way to obtain seat configurations allowing the user to obtain perfect postures.